1. Field of the Invention
The present invention relates to an offset setting system, and more particularly, an offset setting system used when an optical pickup head proceeds a tracking close loop.
2. Description of the Prior Art
Please refer to FIG. 1. FIG. 1 is a schematic diagram that an optical reading/writing apparatus 10 reads/writes an optical storage medium 14 according to the prior art. The optical reading/writing apparatus 10 in the prior art comprises an optical pickup head 12, and reads/writes data on an optical storage medium 14. The optical storage medium 14 comprises a spiral storage track 16. The spiral storage track 16 comprises a series of blocks for storing data. When the optical pickup head 12 reads/writes data on the optical storage medium 14, it reads/writes data on the spiral storage track 16 by locking onto the spiral storage track 16 on the optical storage medium 14.
When the optical reading/writing apparatus 10 uses the optical pickup head 12 to read/write data stored on the optical storage medium 14, the optical reading/writing apparatus 10 rotates the optical storage medium 14, so that the optical pickup head 12 can read/write data on different blocks. Different optical reading/writing apparatuses may have different predetermined reading/writing velocities. Usually “velocity (X)” is used to distinguish different optical reading/writing apparatuses with different predetermined velocities, for instance, a 4X, 16X or 48X optical reading/writing apparatus. The higher the velocity is, the faster the optical pickup head reads and writes. Actually, there are two modes for the velocity of the optical pickup head 12. One is constant linear velocity mode (CLV mode), the other is constant angular velocity mode (CAV mode).
Under the CLV mode, the predetermined velocity of the optical reading/writing apparatus 10 relates to a constant linear velocity. The optical pickup head 12 reads/writes digital data in different blocks on the optical storage medium 14 with a constant linear velocity.
Under the CAV mode, the optical reading/writing apparatus 10 rotates the optical storage medium 14 with a constant angular velocity. That means the optical pickup head 12 reads/writes a circle of data on the optical storage medium 14 with a constant velocity. Thus, when the optical pickup head 12 reads/writes data on an inner circle of the optical storage medium 14, the linear velocity becomes slower. When the optical pickup head 12 reads/writes data in an outer circle of the optical storage medium 14, the linear velocity becomes faster. While the optical pickup head 12 reads/writes data in different blocks on the same circle, the linear velocity is consistent.
No matter under which mode, when the optical pickup head 12 reads/writes data, it often needs to read/write data from/on different data blocks on the optical storage medium 14. When the optical pickup head 12 is in the data block A but needs to read/write data on a target data block C, the optical pickup head 12 needs to jump from the data block A to the data block B first, and then from the data block B to the target data block C. It is because the optical pickup head can only move in a radial direction, and can't jump to the target data block C precisely. After moving to the target data block C, the optical pickup head 12 then can lock on the storage track 16 again and reads/writes data on the target block C.
Although the optical storage medium 14 only has one spiral storage track 16, the optical pickup head 12 seems to cross lots of storage tracks as it moves from the data block A to B. The movement from the data block A to the data block B is called Seek. After seeking, due to jumping over tracks, slight shack or even the optical pickup head 12 deviating from the target storage track may occur. Therefore, the optical pickup head 12 has to return to a steady status before it is available for reading/writing. The status, which is available, is for reading/writing. It is called tracking close loop as the optical pickup head 12 moves from not available to steady and available for reading/writing, as from the data block B to C in FIG. 1. This area from the data block B to the target data block C is called offset. The period, the optical pickup head 12 from ending seeking to be steady for reading data, is called a steady time. Because setting an offset can assist an optical pickup head in reading data on the target data block C steadily, the setting of an offset is very important to the movement of tracking close loop by the optical pickup head. Moreover, when setting the offset length according to the prior art, the offset length is set at a constant value no matter under which mode or what velocity the optical pickup head are.
Please refer to FIG. 2. FIG. 2 is a schematic diagram of an offset from the data block B to the target data block C on the optical storage medium 14 according to FIG. 1. In this example in FIG. 2, there are six data blocks from the data block B to the target data block C. This represents that the offset length is six data blocks.
However, there are some disadvantages for a constant offset length. If the constant offset length is too large, the time that the optical pickup head proceeds a tracking close loop is too long, resulting that the reading/writing time is too long. If the constant offset length is too short, the optical pickup head may exceed the target block but can't read/write data, resulting in data reading/writing failure.
Moreover, if the offset length is constant, the steady time for an optical pickup head reduces as the velocity of the optical pickup head reads/writes increases. For instance, the steady time for the 2X optical pickup head is half of that for the 1X optical pickup head, and the steady time for the 4X optical pickup head is half of that for the 2X optical pickup head. Thus, the optical pickup head 12 must be designed to read/write data faster and more steadily. This causes difficulties in manufacturing and setting an optical pickup head.